1. Field of the Invention
The present invention is directed to a method for controlling the output power of a handheld terminal in a mobile communication system and, more particularly, to a method for controlling the output power of a handheld terminal capable of controlling uplink output power of a handheld terminal in an optimum state.
2. Description of the Background Art
In a mobile communication system, a registered handheld terminal can perform communication by being connected with the other party wirelessly through a base station (BS). One of the methods for making a plurality of handheld terminals communicate simultaneously by using limited frequency or channel resources is known as the time division synchronous code division multiple access (TD-SCDMA) method.
The TD-SCDMA method performs time division of a signal by code division multiple access (CDMA) and is adopted for standard mobile communication system in China.
In a mobile communication system using the TD-SCDMA method, communications are transmitted with the same frequency spectrum from a plurality of handheld terminals in the same geographic area. The transmitted signals are coded according to a unique assigned code. Because they may interfere with each other, signals transmitted from the handheld terminals to the BS in a pertinent service area must have a similar signal intensity. Accordingly, to control the interference level, one of the basic principles in the TD-SCDMA method is that the output power of each of the handheld terminals has to be controlled.
In order to control the output power in the TD-SCDMA method, a closed-loop power control method is used, in which an increase or decrease in the intensity of a signal transmitted from a handheld terminal is commanded based on the intensity of a signal received at the handheld terminal from the BS.
In the china wireless telecommunication standards group (CWTS), for example, a position of a transmit power control (TPC) code that is capable of commanding an intensity increase/decrease of a transmitted signal can be obtained in a time slot burst structure of a physical channel structure.
Hereinafter, a conventional method for controlling the output power of a handheld terminal in a mobile communication system will be described with reference to FIGS. 1 and 2.
FIG. 1 illustrates a structure of a physical channel in accordance with the background art.
As depicted in FIG. 1, in a digital mobile communication system using the TD-SCDMA method, a signal is transmitted by frames. Each frame is a radio frame, which has a time cycle of 10 ms, for example. One frame consists of two sub-frames, each having 6,400 chips and a 5ms cycle, and is thus transmitted at 1.28 MChips/s. Herein, each sub-frame consists of a total of seven time slots from TS0 to TS7. Furthermore, a time slot is used for uplink or downlink communication based on the transmitted data.
Each time slot is 0.675 ms and consists of 864 chips. As shown in FIG. 1, each time slot is constructed in the following order: a data symbol, a first transport format combination indicator (TFCI), a midamble, a synchronization shift (SS) symbol, a TPC symbol, a second TFCI, a data symbol and a guard period (GP).
In this construction, the first and second TFCIs indicate transport formats and data fields. The midamble is a chip sequence which is used by the BS in the uplink or a mobile terminal in the downlink for channel estimation for the retrieval of a user's signals. The GP is inserted at the end of each time slot. The TPC symbol (hereinafter referred to as a TPC) is used for an output power control command, and it is indicated by deviant lines in FIG. 1.
FIG. 2 is a state diagram illustrating an output power control code state of a digital mobile communication system by the TD-SCDMA method in accordance with the background art.
In the TSC103 standard of the CWTS, whether quadrature phase shift keying (QPSK) or 8 phase shift keying (8PSK) is used, the TPC typically performs two commands, such as on the uplink and downlink communications, by being compared with a reference value. More specifically, in order to command an intensity increase/decrease of the transmitted signal, one symbol is allocated to the TPC, two bits are allocated in the QPSK method as a modulation method for a digital signal, and three bits are allocated in the 8PSK method.
However, the TPC uses only two commands for increasing and decreasing output power by using two statuses respectively. In addition, in the conventional terminal transmit path using the above-mentioned system, a physical automatic power control (APC) signal capable of changing only the power in a mixer unit is mainly used. Accordingly, with the above-mentioned output power control method, it is difficult to minimize power consumption or obtain the optimum uplink signal quality in various communication environments.